linux-kernel - Re: [accounting regression since rc1] scheduler updates

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Message-ID: <46C9E4B9.1080900@linux.vnet.ibm.com>
Date:	Tue, 21 Aug 2007 00:30:09 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	schwidefsky@...ibm.com
CC:	Ingo Molnar <mingo@...e.hu>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	heiko.carstens@...ibm.com, Paul Mackerras <paulus@...ba.org>
Subject: Re: [accounting regression since rc1]  scheduler updates

Martin Schwidefsky wrote:
> On Mon, 2007-08-20 at 20:08 +0200, Ingo Molnar wrote:
>> For sched_clock()'s behavior while the virtual CPU is idle: my current 
>> idea for that is the patch below (a loosely analoguous problem exists 
>> with nohz/dynticks): it makes sched_clock() valid across idle periods 
>> too and uses wall-clock time for that.
> 
> Ok, that would mean that sched_clock can just return the virtual cpu
> time and the two hooks starts and stops the idle periods as far as the
> scheduler is concerned. In this case we can use the patch from Jan with
> the new implementation for sched_clock and add the two hooks to the
> places where the cpu-idle notifiers are done (do_monitor_call and
> default_idle). In fact this could be an idle-notifier. Hmm, I take a
> closer look tomorrow when I'm back at the office.
> 

<snip>

I am partially responsible for the regression. While working on the
CPU accounting change, I for some unknown reason always assumed
that sched_clock() was virtualized. I should have taken a closer look.

Ingo, with this new approach, sched_clock() although not virtualized,
advances as if it is (due to the idle state change accounting).
I have one question though, what if the underlying CPU is forcefully
scheduled out from the virtual CPU?

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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